Conventional liquid crystal display elements include, for example, a TN (Twisted Nematic) liquid crystal display element and an STN (Super-Twisted Nematic) liquid crystal display element which use nematic liquid crystals. However, such liquid crystal display elements suffer a drawback of long response time (ms order) to an electro-optic effect when driven at high speed, which causes a disorder in the screen, low contrast, etc., thereby imposing limits on display capacity. To overcome such problems, a liquid crystal display element using a ferroelectric or antiferroelectric liquid crystal has been expected to enter the market for practical applications as the next liquid crystal display forward.
Such ferroelectricity in the liquid crystal is first confirmed by R. B. Meyer, et al., in 1975 by synthesizing a DOBAMBC (2-methylbutyl p-p-(decyloxybenzylidene)-amino! -cinnamate) as a result of research based on the assumption that from the symmetric property of molecules, if an optically-active molecule has a dipole moment in a direction perpendicular to a major axis of the molecule, ferroelectricity would appear in a chiral smectic C phase (SmC* phase) (see R. B. Meyer, L. Liebert, L. Strzelecki and P. Keller:J. Phys. (Paris) 36 (1975) L69).
Here, the structure of the SmC* phase having the ferroelectricity will be explained. In the SmC* phase, the center of gravity of the liquid crystal molecule in the layer is in disorder. However, as shown as cones 101 in a typical depiction of FIG. 6(a), a major axis of the liquid crystal molecule (director 102) is tilted by a prescribed angle .theta. with respect to a layer normal line z (normal line of a layer 103 dividing the smectic layer). The director 102 rotates so as to have a slightly different angle from layer to layer, and thus the liquid crystal molecules have an alignment of a helical structure. In this helical structure, a helical pitch is around 1 .mu.m which is significantly longer than a clearance between layers (around 1 nm). The phase having the described molecular alignment is confirmed not only in the ferroelectric liquid crystal but also in the antiferroelectric liquid crystal (see A. D. L. Chandani, T. Hagiwara, Y. Suzuki, Y. Ouchi, H. Takezoe and A. Fukuda: Jpn. J. Appl. Phys. 27(1988)L729.).
Clark and Lagerwall discovered that such helical structure disappears when the cell thickness is less than around 1 .mu.m (approximately the same as the helical pitch), and as shown in FIG. 6(b), the molecule 104 in each layer is in either one of the bistable modes according to an applied electric field, and they proposed "the surface stabilized ferroelectric liquid crystal (SSFLC)" which is disclosed by Japanese Laid-Open Patent Application No. 107216/1981 (Tokukaisho 56-107216) and U.S. Pat. No. 4,367,924. In FIG. 6(b), the electric field applied to the molecule 104 has a direction perpendicular to the sheet surface of the figure from the back surface side to the front surface side. An electric dipole moment of the molecule 104 is completely arranged in a direction of the applied electric field as shown in each molecule in FIG. 6(b). The above-mentioned mechanism will be explained in reference to FIG. 7. As described, the molecule 104 of the SSFLC formed as a thin cell is in either a bistable mode A or a bistable mode B according to the direction of the applied electric field as shown in FIG. 7. In the bistable mode A shown in FIG. 7, the electric field applied to the molecule 104 has a direction perpendicular to the sheet surface in the figure from the front surface side to the back surface side. While, in the bistable mode B, the electric field has a direction perpendicular to the sheet surface from the back surface side to the front surface side.
Therefore, by forming a SSFLC cell between two polalizers whose polarization axes cross at right angle, for example, in such a manner that the major axis of the molecule lies parallel to the direction of one polarizer in the bistable mode B (direction 111 shown by an arrow in the figure), a bright state appears in the bistable mode A by allowing a transmission of light, while a dark state appears in the bistable mode B by shutting off a transmission of light. Namely, by switching the direction of the applied electric field, a black-and-white display can be achieved. Here, an apparent angle formed by an optical axis in the state where the molecule is in one bistable mode (for example, the bistable mode A) and an optical axis in the state where the molecule is in the other bistable mode (for example, the bistable mode B) is referred to as "memory angle".
In the SSFLC, since a spontaneous polarization and the electric field interact directly, different from the general switching using the dielectric anisotropy in the nematic crystal, a short response time of not more than millisecond (ms) order can be achieved with respect to the electric field. Besides, the SSFLC offers a beneficial feature of memory function that once the bistable mode is achieved, the bistable mode can be maintained even with a removed electric field, thereby eliminating the necessity of applying a voltage constantly.
As described, in the SSFLC, by utilizing its advantageous characteristic of short response time and memory function, the display signal can be written per each scanning line at high speed, thereby permitting a display of a large capacity with the simple matrix drive system. For the described beneficial characteristics, the application of the SSFLC to a hung-wall type television is also expected.
The liquid crystal molecule in the described SSFLC preferably has a uniform alignment of the bookshelf structure wherein a liquid crystal layer 120 is formed perpendicularly to a glass substrate 121 as shown in FIG. 8(a). In practice, however, the liquid crystal molecule has an alignment of the chevron structure wherein a liquid crystal layer 120 is bent to the normal line of a glass substrate 121 in chevron as shown in FIG. 8(b). The liquid crystal molecule having an alignment of the chevron structure has drawbacks in that an memory angle is reduced which causes the amount of transmitted light to be reduced, or an alignment defect, called "zigzag defect" occurs between the layers which are bent in the opposite direction, resulting in low contrast.
As a solution to such problems, a method of approximating the structure of the liquid crystal layer to the quasi-bookshelf structure while applying an electric field in the SmC* phase has been proposed, for example, by Y. Sato (see "Japanese Journal of Applied Physics" Vol. 28, No. 3, March, 1989, pp.L483-L486). This method is effective in increasing the memory angle, yet has such drawbacks that the irregularity in texture tends to occur due to many line defects, or the response time to the drive electric field increases, etc. Therefore, the described method, in fact, does not provide an effective solution to the described problem.